1. Field of the Invention
The present invention relates to a carbon film having a shape suitable for performing field emission.
2. Description of the Related Art
It is known that field emission can be expressed by the Fowler-Nordheim equation describing density of current emitted to vacuum. The equation is given by the following.I=sAF2/φexp(−B3/2/F)F=βV
where I denotes field emission current, s denotes field emission area, A denotes constant, F denotes field strength, φ is work function, B is constant, β is field concentration coefficient, and V is application voltage.
The field concentration coefficient β is a coefficient for converting the application voltage V to the field strength F(V/cm) in accordance with the shape of the tip portion and the geometric shape of a device.
The smaller the work function φ of a material is and the larger the field concentration coefficient β is, the stronger the field emission current I becomes, and the field emission current I increases.
Electrons are confined in a solid body by a potential barrier expressed as the work function φ. When the electric field is strongly concentrated on the surface of the solid body and the potential barrier becomes thinner to about 1 nm or less, the probability that electrons are emitted from the solid body to vacuum by the tunneling phenomenon due to the wave nature of electrons sharply increases.
The phenomenon that electrons are emitted to vacuum due to the field concentration is called field emission. The field emission current I can be obtained by integrating the product between the incidence density of electrons which collide with the potential barrier and the probability that the electrons tunnel the potential barrier with an overall energy region.
The Fowler-Nordheim equation shows the above. As a structure of performing such field emission, for example, a Spindt-type field emission structure in which a small conical shape is formed by silicon or metal is known.
However, in the Spindt type, the height of the tip is limited so that it is difficult for the Spindt type to address an improvement in the field emission characteristic.
To solve the drawback of the Spindt type, a carbon nanotube having a high aspect ratio is being developed. The carbon nanotube is obtained by forming a carbon film in a needle shape by performing the chemical vapor deposition (CVD) or the like. The carbon nanotube is extremely narrow and long, the radius “r” of curvature of the tip is smaller than that of the Spindt type, the field concentration coefficient β increases, and the field emission characteristic becomes excellent.
In the case of a carbon nanotube, however, at the time of increasing the application voltage to increase the field emission current I, after the voltage exceeds an application voltage, the field emission current I does not increase and is saturated.
Consequently, in the case of using a carbon nanotube as an electron emission source for various devices, apparatuses, and the like, for example, in the case of using a carbon nanotube for a field-emission-type illuminating lamp, at the time of adjusting the right emission brightness by adjusting application voltage, the adjustment range is extremely regulated.
In a carbon nanotube, the aspect ratio as a ratio of height to diameter is extremely high, so that the heights of tips tend to vary and are not easily aligned. Further, since the tips are not easily aligned and it is hard to mechanical support the carbon nanotube on a substrate, stability is missing. It is difficult for a carbon nanotube to come into electric contact with a substrate for passing current. When a number of carbon nanotubes are provided at high density, field concentration is suppressed and the electron emission characteristic easily deteriorates.